It is widely known that much of the art that graced games of the 1960s and 1970s derived from prior sources. The original cover of Dungeons & Dragons came from a panel of the comic book Strange Tales #167 (which I reproduce in Playing at the World). Don Lowry, who published the Guidon Games line of rules and frequently provided cover illustrations for the International Wargamer, also drew on pre-existing pictures: as we see here in perhaps his single most famous drawing, the cover of Chainmail. The original appeared on page 114 of Jack Coggins's self-illustrated The Fighting Man (1966), at the start of the chapter "Crescent and Cross" about the Crusades. Gygax mentions this volume in Domesday Book #7.
Zenopus in his post on the subject noted that a version of this image drawn by Gygax appears in the Panzerfaust for Novemebr of 1970, along with his"Crusadomancy" variant. That variant had earlier appeared in Domesday Book #5, and with it a fuller version of this illustration than we see in Panzerfaust: and in this one, we can clearly see "After Coggins" written in the corner.